Charged particle beam adjusting method and charged particle beam apparatus

ABSTRACT

In an apparatus for obtaining an image by irradiating a charged particle beam on a specimen, a condition of the beam conditioned differently from vertical incidence as in the case of the beam being tilted is required to be adjusted. To this end, the apparatus has a controller for automatically controlling a stigmator, an objective lens and a deflector such that astigmatism is corrected, focus is adjusted and view filed shift is corrected. The controller has a selector for inhibiting at least one of the astigmatism correction, focus adjustment and FOV shift correction from being executed.

BACKGROUND OF THE INVENTION

The present invention relates to a method of adjusting a beam conditionfor a charged particle beam and a charged particle beam apparatus andmore particularly, to a method of making astigmatism correction, focuscorrection and FOV (field of view) shift correction when giving the beama tilt.

In the charged particle beam apparatus represented by a scanningelectron microscope, a charged particle beam is focused to a fine beamwhich in turn is scanned on a specimen to obtain desired informationfrom the specimen (for example, a specimen image). In the chargedparticle beam apparatus as above, the resolving power becomes higheryear by year and besides, recently, there is a growing need forobtaining a tilt image of a specimen by tilting a charged particle beamin relation to the specimen. This is accounted for by the fact that foracquisition of tilt images of the specimen, it is general to tilt aspecimen stage but for the purpose of preventing a FOV shift at highmagnification and acquiring a tilt image of the specimen more speedily,giving the charged particle beam a tilt in relation to the specimen ismore rational than mechanically tilting the specimen stage.

A technique for irradiating a beam while tilting it is known asdescribed in Patent Document 1 (JP-A-55-048610 (U.M.)) or PatentDocument 2 (U.S. Pat. No. 4,983,832 corresponding to JP-A-02-033843),for instance. In the known technique, a charged particle beam is causedto be incident on a region which is out of axis of an objective lens andis then tilted using the focusing action (Imaging Action) of theobjective lens.

SUMMARY OF THE INVENTION

The above prior art describes nothing about astigmatism correction,focus correction or FOV shift correction required to be dealt with whenthe beam is tilted. During the beam tilting, specific problems arecaused which do not take place when the beam is incident on the specimenvertically thereof.

An object of this invention is to provide beam condition adjustingmethod and apparatus suitable for adjusting a beam condition when a beamis placed in condition different from vertically incident beamcondition, as in the case of the beam tilting.

Firstly, according to the present invention, to accomplish the aboveobject, a charged particle beam apparatus is provided which comprises acharged particle beam source, an objective lens for focusing a chargedparticle beam emitted from the charged particle beam source to irradiateit on a specimen, a detector for detecting charged particles emittedfrom the specimen, a deflector for deflecting the charged particle beamto a region which is out of axis of the objective lens to thereby tiltthe charged particle beam in relation to an optical axis of theobjective lens, a stigmator for correcting an astigmatic aberration ofthe charged particle beam, and a controller for automaticallycontrolling the stigmator, objective lens and deflector such that whenthe charged particle beam is tilted, astigmatism correction and focusadjustment of the charged particle beam and FOV shift correction areconducted on the basis of the charged particles detected by thedetector, wherein the controller includes selection means for inhibitingat least one of the astigmatism correction, focus adjustment and FOVshift correction from being carried out. According to the firstconstruction, when the beam is tilted, an operator can set a correctioncondition at will in the light of his or her empirical rule byrespecting achievement of high accuracy, achievement of low damage tospecimen and achievement of speedup of processing speed which can bebrought about by the beam condition correction.

Secondly, method and apparatus are provided in which when a chargedparticle beam is deflected from an optical axis of an objective lens forfocusing the charged particle beam and is irradiated while being tiltedin relation to the objective lens optical axis, pattern matching isconducted on the basis of comparison of a template based on an imageacquired before the tilting with an image acquired after the tilting tocorrect a FOV shift for the charged particle beam on the basis of aresult of the pattern matching. According to the second construction,even when the field of view is shifted owing to correction of the beamcondition during the beam tilting, the shift can be corrected properly.

Thirdly, method and apparatus are provided in which when a chargedparticle beam is deflected from an optical axis of an objective lens forfocusing the charged particle beam and is irradiated while being tiltedin relation to the objective lens optical axis, a stigmator capable ofadjusting intensities of astigmatism correction in a plurality ofdirections is used to determine evaluation values in respect ofindividual combinations of adjusting intensities in the plurality ofdirections and on the basis of a determined combination of adjustingintensities for which the evaluation value is high, a combination ofadjusting intensities of the stigmator is settled. According to thethird construction, an astigmatic aberration when the beam is tilted canbe corrected properly.

Fourthly, method and apparatus are provided in which when a chargedparticle beam is deflected from an optical axis of an objective lens forfocusing the charged particle beam and is irradiated while being tiltedin relation to the objective lens optical axis, focus adjustment isconducted after the beam tilting and thereafter FOV shift correction ismade. According to the fourth construction, when the beam is tilted,highly accurate FOV shift correction can be made on the basis of animage in exact focus.

As described above, according to this invention, beam conditionadjusting method and apparatus can be provided which are suitable foradjusting the beam condition especially during the beam tilting. Otherconstructions and advantages of the present invention will becomeapparent when reading embodiments of the invention to be describedhereinafter with reference to the accompanying drawings.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the construction of a scanningelectron microscope according to an embodiment of the invention.

FIG. 2 is a diagram useful to explain an example in which an electronbeam is irradiated on a specimen while being tilted through the use ofan image shift deflector.

FIG. 3 is a diagram showing a GUI screen for setting conditions when theelectron beam is irradiated while being tilted.

FIG. 4 is a flowchart useful to explain processing steps in makingautomatic astigmatism correction, automatic focus correction and FOVshift correction.

FIG. 5 is a flowchart useful to explain details of the automaticastigmatism correction (automatic stigmatization).

FIG. 6 is a diagram showing an example of a stigmator.

FIGS. 7A and 7B are diagrams showing an example of acquiring evaluationvalues of the stigmator two-dimensionally.

FIG. 8 is a flowchart useful to explain details of automatic focuscorrection (auto-focus).

FIG. 9 is a flowchart useful to explain details of FOV shift correction(centering).

FIG. 10 is a flowchart useful to explain an example of preparingexpressions for correcting the amount of focus adjustment and the amountof FOV shift correction in relation to tilt angles.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring first to FIG. 1, the construction of a scanning electronmicroscope according to an embodiment of this invention will bedescribed. A voltage is applied across cathode 1 and first anode 2 bymeans of a high voltage control power supply 21 controllable by acontrol and operating device 30 (control processor) and a predeterminedamount of emission current is drawn out of the cathode 1. Since anaccelerating voltage is applied across the cathode 1 and a second anode3 by means of the high voltage control power supply 21 controlled by thecontrol and operating device 30, an electron beam 4 emitted from thecathode 1 is accelerated to travel toward a succeeding lens system. Theelectron beam 4 is condensed with a condenser lens 5 controlled by acondenser lens control power supply 22 and its unnecessary region iseliminated by means of an aperture plate 8. Subsequently, the electronbeam is focused into a small spot on a specimen 9 by means of anobjective lens 7 controlled by an objective lens control power supply 23and scanned two-dimensionally on the specimen by means of a deflector11. Near the deflector 11, an image shift deflector (not shown) isarranged which is adapted for deflecting the electron beam 4 from itsoptical axis (the trajectory of the electron beam 4 when not beingaffected by deflection).

By using the image shift deflector, the scanning position of electronbeam 4 can be changed in relation to the specimen 9. In addition, bydeflecting the electron beam from an optical axis of the objective lens7, the angle at which the electron beam 4 is irradiated on the specimen7 can be changed. In the present invention, an example of changing thedeflection angle of electron beam 4 through the use of the image shiftdeflector will be described but this is not limitative and for example,the electron beam may be deflected with another type of deflector. Theapparatus according to the present embodiment is also provided with astigmator (not shown) to be described later.

A scanning signal of the deflector 11 is controlled in accordance withobservation magnifications by means of a deflector control power supply24. The specimen 9 is fixedly mounted on a specimen stage 41 which ismovable two-dimensionally. The movement of the specimen stage 41 iscontrolled with a stage controller 25. Secondary electrons 10 generatedfrom the specimen 9 under irradiation of the electron beam 4 aredetected by a secondary electron detector 12 and a drawing unit 28controls detected secondary signals such that they are converted intovisual signals and are suitably arranged on a different plane, therebyensuring that an image corresponding to a surface shape of the specimencan be displayed as a specimen image on a specimen image display unit26.

An input device 27 is adapted to provide an interface between anoperator and the control and operating device 30 and the operatorcontrols the aforementioned units, designates a measure point andcommands dimensional measurement through the medium of the input device27. To add, the control and operating device 30 is provided with amemory unit not shown so that obtained measure values and controlconditions for the individual units may be stored in the memory unit.

The signals detected by the secondary electron detector 12 are amplifiedby a signal amplifier 13 and thereafter accumulated in an image memoryinside the drawing device 28. It will be appreciated that the apparatusof this embodiment is provided with the secondary electron detector 12but this is not limitative and a backscattered electron detector fordetection of backscattered electrons or a detector for detection of raysof light, electromagnetic waves or X-rays may be provided insubstitution for the secondary electron detector or in combinationthereof.

Address signals corresponding to memory positions of the image memoryare generated in the control and operating device 30 or in a separatelyinstalled computer and then converted into analog signals which are inturn supplied to the deflector 11. In case the image memory has, forexample, 512×512 pixels, an address signal in X direction is a digitalsignal repeating from 0 to 512 and an address signal in Y direction is adigital signal which repeats from 0 to 512 and is added with +1 at thetime that the address signal in X direction starting from 0 reaches 512.These digital signals are converted into analog signals. Since anaddress of the image memory corresponds to an address of a deflectionsignal for scanning the electron beam, a two-dimensional image of aregion within which the electron beam is deflected by the scanning coilis recorded in the image memory. To add, signals in the image memory canbe read out sequentially on time series base by means of a read-outaddress generating circuit synchronized with a read clock. A signal readin correspondence with an address is converted into an analog signal toact as a brightness modulation signal for the specimen image displayunit 28.

The image memory has the function to store images (image data) byoverlapping (synthesizing) them for the purpose of improving the S/Nratio. For example, by storing images obtained through 8 two-dimensionalscanning operations in the overlapping fashion, one complete image canbe formed. In other words, images formed in a unit of one or more X-Yscanning operations are synthesized to form an ultimate image. Thenumber of images for forming one complete image (frame cumulativenumber) can be set as desired and a proper value can be set by takinginto account such a condition as secondary electron generationefficiency. By further overlapping plural images each formed byintegrating a plurality of images, an image desired to be obtainedultimately can also be formed. At the time that a desired number ofimages are stored or after that, blanking of the primary electron beammay be executed to interrupt input of information to the image memory.

Where the frame cumulative number is set to 8, sequence may be set inwhich at the time that a ninth image is inputted, the first image iserased and as a result, 8 images remain or alternatively, weightedsumming mean may be carried out in which when a ninth image is inputted,accumulated images stored in the image memory are multiplied by ⅞ andthe product is added with the ninth image.

Also, the apparatus according to the present embodiment has the functionto form a line profile on the basis of detected secondary electrons orbackscattered electrons. The line profile is formed on the basis ofamounts of detected electrons or brightness information of specimenimages detected when a primary electron beam is scanned linearly ortwo-dimensionally and the thus obtained line profile is used for, forexample, dimensional measurement of a pattern formed on a semiconductorwafer.

In the dimensional measurement of a pattern, two vertical or horizontalcursor lines are displayed, along with a specimen image, on the specimenimage display unit 26, the two cursors are located at two edges of thepattern by manipulation through the input device 27 and on the basis ofinformation of a magnification of an image of the specimen and adistance between the two cursors, the control and operating device 30calculates a measurement value as a dimensional value of the pattern.

In the description given in connection with FIG. 1, the controlprocessor (control and operating device) is described as being integralwith the scanning electron microscope or equivalently thereto but thisis in no way limitative and a process to be described below may becarried out with a control processor installed separately from thescanning microscope column. In that case, there need a transmissionmedium for transmission of detection signals detected by the secondaryelectron detector 12 to the control processor and for transmission ofsignals from the control processor to the lenses and deflector ofscanning electron microscope and input/output terminals for input/outputof the signals transmitted via the transmission medium. In analternative, a program for execution of the process to be describedbelow may be registered in a memory medium and the program may beexecuted by a control processor having an image memory and adapted tosupply necessary signals to the scanning electron microscope.

Further, the apparatus according to the present embodiment has thefunction to precedently store, as a recipe, conditions (measure sites,optical conditions of the scanning electron microscope and the like) forobservation of, for example, a plurality of points on a semiconductorwafer and conduct measurement and observation in accordance with thecontents of the recipe. Furthermore, a program for performing theprocess to be described below may be registered in a memory medium andthe program may be executed by a control processor having an imagememory and adapted to supply necessary signals to the scanning electronmicroscope. More specifically, embodiments of the invention to bedescribed hereinafter stand as inventions of a program adoptable in acharged particle beam apparatus such as a scanning electron microscopeprovided with an image processor.

Turning now to FIG. 2, an example will be described in which an electronbeam 4 is irradiated on a specimen 9 while being tilted through the useof a deflector arranged above the objective lens 7. The electron beam 4can be tilted by means of the previously-described image shift deflectorbut any types of coil having ability to take a similar deflective actionmay be employed. The electron beam 4 is deflected from the electron beamoptical axis by means of the deflector 51 and is caused to be incidenton a lens principal plane 52 of objective lens 7 obliquely thereto. Theelectron beam 4 being incident on the lens principal plane 52 isaffected by back deflective action of the objective lens 7 so as to bedeflected toward the electron beam optical axis.

When the electron beam is irradiated by giving it a tilt, the locus ofthe beam deviates from the electron beam optical axis representing anoriginal ideal locus, as designated by 53, giving rise to out-of-axischromatic aberration and coma of the objective lens 7. In addition tothe aberration of objective lens 7, errors in distance between deflector51 and objective lens 7 due to mechanical assembling errors areinvolved, so that the tilted electron beam 4 will be irradiated at asite different from the optical axis 53 on the specimen 9 and the fieldof view of observation position is shifted. The out-of-axis chromaticaberration, coma and FOV shift (out of centering) caused in the courseof tilting of the electron beam fatally impair the performance theapparatus for observing a desired position with high resolving power hasoriginally. Embodiments of the invention to be described hereinaftersolve the problems encountered when the electron beam is irradiatedwhile being tilted.

Embodiment 1

A GUI (graphical user interface) screen used to set conditions forirradiating an electron beam while giving it a tilt is illustrated inFIG. 3. The control and operating device 30 has a program for displayingsuch a picture on the specimen image display unit 26. On the basis ofinformation set on the GUI screen, the aforementioned control andoperating device 30 calculates current or voltage supplied to thedeflector 51.

A viewing point adjustment picture 101 is for setting irradiationdirection and irradiation angle of the electron beam. By adjusting aviewing point position 102 on the viewing point adjustment picture 101,an irradiation direction of the electron beam and an irradiation anglethereof can be set. A tilt angle and a tilt direction are displayed at atilt angle value indicator 103 and a tilt direction indicator 104,respectively, on the basis of the viewing point setting on the viewingpoint adjustment picture 101. Alternatively, the tilt angle and tiltdirection may be inputted directly, for instance. Provided beneath theviewing point adjustment picture 101 are check buttons for selecting aprocess to be performed during the electron beam tilting. A first checkbutton 105 is for selecting whether an automatic astigmatism correctionis to be made or not, a second check button 106 is for selecting whetheran auto-focus correction is to be made or not and a third check button107 is for selecting whether a centering function of FOV (field of view)is to be made or not.

The focus correction, astigmatism correction and FOV shift correction inthe scanning electron microscope, for instance, are carried out on thebasis of detection of electrons emitted or discharged from the specimen.For example, in the case of FOV shift correction, how much a featuredobject is moved between an image before a FOV shift and an image afterit is decided by analyzing images formed on the basis of detection ofsecondary electrons. Making a decision on the basis of image processingneeds to acquire images for decision, thus requiring a slight processingtime and has some fear for damaging a specimen depending the kind of thespecimen.

Structurally, the present embodiment can automatically perform allprocesses of automatic astigmatism correction, automatic focuscorrection and FOV shift correction during the electron beam tilting,wherein a means is provided which selects process by process whether theprocesses are to be carried out or not. Accordingly, the operator canselect a processing item at will by taking damages to the specimen and aprocessing time into account. In other words, in an apparatuspresupposing execution of the aforementioned three processes, theapparatus has the function to make such setting that at least one ofthese processes is not done selectively to enable the operator to makesetting at will in the light of his or her empirical rule by respectingachievement of high accuracy, achievement of reduction of damage to thespecimen and achievement of speedup of processing speed which can bebrought about by the beam condition correction.

Also, the operator can set the necessity or non-necessity of beamadjustment while checking the beam for its tilt angle and tiltdirection. Further, the operator can decides necessity or non-necessityof the above adjustments while visually grasping the degree of a tiltgiven to the beam. For example, when the beam is tilted, there is apossibility that the field of view is shifted as the focus changes. Incase the tilt angle is small, the field of view sometimes hardly shiftseven as the focus adjustment proceeds. The operator can afford to set atwill necessity or non-necessity of FOV shift correction in accordancewith the tilt angle and the necessity or non-necessity of focusadjustment.

Referring now to FIG. 4, there is shown a flowchart for explainingprocess steps when making the automatic astigmatism correction,automatic focus correction and FOV shift correction during the electronbeam tilting. When execution of beam tilt is selected on a programexecuted by the control and operating device 30 at the time that thefield of view is positioned to an electron beam irradiation position ona specimen, a beam tilt process is started. The beam tilting isconducted by deflecting an electron beam from the electron beam opticalaxis through the use of the deflector 51 such as an image shiftdeflector as has been explained in connection with FIG. 2. In thecontrol and operating device 30, current or voltage values supplied tothe deflector are stored in respect of individual angles settable on theGUI screen as shown in FIG. 3.

Before tilting the beam, an image A is acquired (S0001). This image willbe formed into a template for use in pattern matching to be describedlater. Next, the magnification is lowered (S0002) and an image B isacquired (S0003). The image obtained in the step S0003 is also formedinto a template for the later pattern matching. Subsequently, whilekeeping the magnification lowered, the electron beam is tilted (S0004).In this phase, an image C after tilting is acquired (S0005) and patternmatching is carried out between the image C (beam tilt image) and theimage B (none beam tilt image) acquired in the step S0003 to make an FOV(field of view) shift correction (S0006).

In the present embodiment, pattern matching is effected between theimages obtained at different beam tilt angles to ensure that a shift ofa view field after tilting from a view field before tilting can becorrected. With the FOV shifted through the beam tilting, there is apossibility that a measure object pattern is moved out of the field ofview. In the present embodiment, the magnification is once loweredbefore beam tilting and a template of an image at a low magnification isformed. Then, this template for pattern matching is compared with animage obtained after beam tilting to correct a FOV shift. With thisconstruction, even when the field of view is shifted to a large extentby the beam tilting, a pattern to be compared with the template will notgo out of sight and the FOV shift correction can be permitted.

Next, the control and operating device 30 decides, on the GUI screenexplained in connection with FIG. 3, whether the first check button 105(automatic astigmatism correction) is selected. With the first checkbutton 105 selected, a process in step S0007 is carried out. Similarly,automatic focus correction (S0008) is carried out when the second checkbutton 106 is selected.

Next, a signal supplied to the deflector is changed in order that thesame range as that for the image A can be scanned with the electron beam(at the same magnification as that for the image A) (S0009) and at thattime, an image D is obtained (S0010). This image D is one acquired afterthe astigmatism correction and focus correction have been made andtherefore, a FOV shift correction (S0011) based on pattern matchingbetween the image D and the image A can be effected with high accuracy.The FOV shift correction (S0011) is conducted when the third checkbutton 107 is selected.

As in the present embodiment, by letting the apparatus for makingautomatic astigmatism correction, automatic focus correction andautomatic FOV shift correction have the function to make such settingthat at least one or more of these processes are selectively preventedfrom being executed, compatibility between the throughput and the highlyaccurate measurement can be assured as described previously.

In the present embodiment, the processes of automatic astigmatismcorrection (S0007), automatic focus correction (S0008) and FOV shiftcorrection (S0011) are sequentially executed in this order as describedpreviously. Generally, after positioning an observation object and thelike at the center of field of view, the astigmatism correction andfocus correction are carried out. But when the objective lens conditionis changed while keeping the beam tilted, there is a possibility thatthe specimen image will be shifted. Accordingly, in the presentembodiment, the FOV shift correction is conducted after the astigmatismcorrection and focus correction have been made. Through this, even ifthe image is shifted as being affected by the astigmatism correction andfocus correction, the image shift can be corrected. By defining thesteps in this manner, the observation object can be prevented from goingout of sight even when focusing is adjusted during the beam tilting andhence the astigmatism correction and focus correction during the beamtilting can be automated.

In the present embodiment, approximate positioning is completed using animage at a low magnification and thereafter a highly accurate specimenimage is formed by adjusting the beam, followed by ultimate positioningusing an image at a high magnification, and consequently automatic andhighly accurate FOV shift correction can be materialized with ease.

Embodiment 2

The automatic astigmatism correction (automatic stigmatization)explained in embodiment 1 will be detailed using a flowchart shown inFIG. 5. The stigmator is comprised of, for example, a multi-pole coil asshown in FIG. 6 and is interposed between cathode 1 and objective lens7. The stigmator to be explained with reference to FIG. 6 is atwo-dimensional stigmator in which astigmatic aberration correctingintensities in respective directions can be adjusted by adjustingsignals stx and sty applied to the respective coils so as to correct anastigmatic aberration. In the present embodiment, astigmatism correctionwill be described by way of example of the two-dimensional astigmatismcorrection but this is not limitative and for example, a stigmator formaking three-dimensional astigmatic aberration correction can also beapplicable to the present embodiment.

Firstly, a magnification during astigmatism correction is acquired(S0012). If the magnification value is higher than a magnification upperlimit A, this magnification value is automatically changed and set tomagnification A (S0013, S0014) but if lower than a magnification lowerlimit B, this magnification value is automatically changed and set tomagnification B (S0015, S0016). In making the astigmatism correction,there arises a problem that if the magnification is too low, the picturequality changes dully even when the lens intensity of objective lens ischanged and as a result a sharp peak cannot be found in evaluationvalues plotted in relation to lens intensity values. Conversely, themagnification set to an excessively high value gives rise to a problemthat when the astigmatism is changed by means of the stigmator, there isa possibility that a pattern on the specimen moves out of the screen andthe pattern edge necessary for observation of changes in image qualitycannot remain sufficiently in the field of view. In such an event, theimage quality can hardly be evaluated correctly in the course ofchanging of the astigmatism.

Hence, according to the present embodiment, in the event that themagnification deviates from values within a predetermined range beforemaking an astigmatism correction, the magnification is corrected topermit a proper astigmatism correction to be made. There is apossibility that such a problem as above will also arise during focusadjustment and therefore, the magnification may be restricted as aboveduring focus adjustment.

Subsequently, the signals stx and sty are changed in combination toobtain evaluation values two-dimensionally as shown in FIG. 7A. In anexample shown in FIG. 7A, evaluation values for 3×3=9 sites can beacquired. The evaluation value can be determined by image-processing thesharpness (abruptness of a change in contrast) at an edge portionappearing in the field of view. More specifically, signals stx and styare deflected from a position P corresponding to the signals stx and styat present to obtain images corresponding to individual differentcombinations of stx and sty (S0017). Evaluation values are acquired inrespect of these images to obtain two-dimensional evaluation results(S0018).

In case a combination of stx and sty exhibiting a high evaluation value(peak evaluation value) corresponding to the position P is found fromthe obtained two-dimensional evaluation results, the combination of stxand sty is set as a proper astigmatism correction amount (S0019).Further, by approximating evaluation values acquired two-dimensionallyat the 9 sites with a two-dimensional Gaussian curve, a more accurateastigmatism correction amount may be obtained.

To the contrary, when a peak evaluation value exists at an edge of anarea where evaluation values are obtained two-dimensionally as shown inFIG. 7B, points getting outwardly clear of that point must be sampled.If the result shows that the point, designated at P, exhibits thehighest evaluation value, a combination of stx and sty corresponding tothe point P is set as a proper astigmatism correction amount. Inaddition, by approximating evaluation values acquired two-dimensionallyat 9 sites around the center point P with a two-dimensional Gaussiancurve as described previously, a more accurate astigmatism correctionamount may be obtained.

If a position at which the evaluation value is higher than that at thepoint P exists when the position is kept clear of the point P outwardly,the clear off position or point can impersonate a new P point and pointsaround it can be sampled to determine an optimum astigmatism correctionamount in a similar way.

As described above, the proper astigmatism correction amount can besettled. According to the present embodiment, the astigmatic aberrationduring the beam tilting can be corrected with high accuracy. With thebeam tilted, the electron beam travels along a locus out of axis of theobjective lens to generate an astigmatic aberration the amount of whichis larger than that of an astigmatic aberration caused when the beam isvertically incident. For this reason, if only one of the signals stx andsty (for example, only stx) is changed, the evaluation value is anyhowreduced when a large deviation takes place in the direction (forexample, sty) in which the signal is not changed and there is apossibility that any peak of evaluation values cannot be presumed highlyaccurately. According to this embodiment, even when a relatively largeastigmatic aberration takes place, an optimum correction amount can bedetermined with high accuracy by changing both the stx and stysimultaneously and then finding evaluation values.

Further, by measuring in advance an astigmatic aberration taking placewhen giving the beam a tilt and approximately estimating a deformdirection and a deform amount of the beam to precedently set astigmatismcorrection amounts to some extent, time required for determining anoptimum astigmatism correction amount (sampling time for determining theoptimum astigmatism correction amount) can be reduced drastically.

Embodiment 3

The automatic focus correction explained in connection with embodiment 1will be detailed with reference to a flowchart of FIG. 8. When the lensintensity of objective lens is changed while placing the beam in tiltcondition, the field of view moves. This is because the beam tilt causesan x-y direction component (when the objective lens optical axisdirection is defined as z direction) to be included in the direction inwhich focus is to be adjusted. Described in the present embodiment is afocus adjustment method and the construction therefor which can conductproper focus adjustment even when the view field movement occurs.

Firstly, at a beam not placed in tilt condition (top-down image) thecenter of FOV is set to a suitable pattern, in the FOV, then focus isadjusted using the pattern. Next, a desired beam tilt angle is set in adesired beam tilt direction (S0020). The beam tilt direction and beamtilt angle correspond to those set on the GUI screen of FIG. 3. In thisphase, the lens intensity of objective lens is adjusted to make matchingwith an exact focus. An amount of change of lens intensity (inclusivesign) and an amount of FOV shift (inclusive of direction) at that timeare measured. This work is done at individual angles in individual beamtilt directions. This work may also be done in respect of allcombinations of beam tilt directions and beam tilt angles oralternatively may be done by approximating, in terms of expression, partof combinations to presume remaining combinational conditions.

The FOV shift and focus offset in case of beam tilt described in aboveare added as correction values in advance when giving the beam a tilt.The FOV shift amount can be adjusted by adjusting the image shiftdeflector or movement of the stage. This can prevent the focus and viewfield from shifting to a great extent during beam tilting and time ofsucceeding focus adjustment work can be shortened. Next, the lensintensity is fluctuated to determine a more suitable focal position. Inthis phase, the FOV shift correction amount is corrected by means of theimage shift deflector in accordance with the previously-determinedrelational expression between lens intensity change and FOV shiftamount. In this manner, the FOV shift can be reduced even in the focusadjustment during the beam tilting without degrading the accuracy offocus adjustment. The focus adjustment is carried out by fluctuating theobjective lens condition in plural steps so as to acquire images,selecting an image of high focus evaluation value from the acquiredimages and selecting an objective lens condition used for forming animage for which the evaluation value is high.

Within a fluctuation width of objective lens condition set in stepS0021, steps S0022 to S0026 are repeated predetermined times (S0027) andan objective lens condition for the highest focus evaluation value isdetected from the conditions obtained through the steps S0022 to S0027.If any peak cannot be found within the fluctuation width of lenscondition (in the case of the evaluation value increasing monotonouslyor decreasing monotonously), there is a possibility that the rangewithin which the objective lens condition is fluctuated is unsuitableand therefore, the fluctuation range is reset (S0028) and then an exactfocal position of objective lens condition is again detected. Theobjective lens condition settled in this manner is set as an optimumlens condition (S0029).

With the above construction, even when the objective lens condition ischanged in performing automatic focus adjustment on the basis of imageevaluation, images can be evaluated in the same specimen area to permithighly accurate focus adjustment.

In the case of a magnetic objective lens using a ferromagnetic material(for example, iron core coil), a given magnetic field is sometimesgenerated in a short time after current is passed through the lens sothat a desired focus condition may be obtained. This phenomenon iscalled a magnetic aftereffect and because of the magnetic aftereffect,movement of the view field sometimes keeps continuing for a while afterthe focus adjustment and images are drifted during image accumulation toblur an ultimate image.

In the present embodiment, even when the focus adjustment needs to beeffected plural times in the same view field as in the case of athree-dimensional structure, images are not acquired before the focuscondition is stabilized following the focus adjustment for the purposeof suppressing image blur of a cumulated image, thereby permittingacquisition of a cumulative image removed of image blur.

To add, a technique is available which forms an electrostatic lens byapplying a negative potential to the specimen and/or applying a positivepotential to the electron beam passage of the objective lens. Thus, whenthe focus adjustment needs to be done for the same field of view pluraltimes in the course of beam tilting, for example, the focus adjustmentmay be done by selectively using the electrostatic lens to solve theabove problem.

Embodiment 4

The FOV shift correction (centering) carried out in the steps S0006 andS0011 in the flowchart shown in FIG. 4 will be described in greaterdetail with reference to a flowchart of FIG. 9. In step S0030, atemplate is formed on the basis of an image acquired in the step S0001or S0003 in FIG. 4. The beam used in this phase is one irradiated in thesame direction as the electron beam optical axis or in the directionvertical to the specimen surface direction.

Next, pattern matching is carried out between an image after beam tiltacquired in the step S0005 or S0010 and the template formed in the stepS0030 (S0031). On the basis of this pattern matching, a shift amountbetween the image before tilting and the image after tilting is acquired(S0032) and view field correction based on the image shift is conductedon the basis of the shift amount (S0033). This FOV shift correctioncorresponds to the step S0006 or S0011 in FIG. 4.

By performing the pattern matching through the use of images before andafter the beam tilting as in the present embodiment, a FOV shiftattributable to the beam tilting can be corrected. Further, according tothe flowchart shown in FIG. 4, images are acquired at a highmagnification and a low magnification before the beam is tilted to formtemplates and the FOV shift correction is made on the basis of thesetemplates. According to these steps, even when the field of view isshifted to a great extent depending on the beam condition adjustmentduring the beam tilting, the beam condition can be adjusted whilecorrecting the FOV shift properly.

In addition, when images for FOV shift correction are acquired (S0010)after the beam condition adjustment (S0007, S0008) has been completed, aFOV shift based on images acquired under good condition can be detected,thereby ensuring that the FOV shift correction can be done with highaccuracy.

Embodiment 5

In the example shown in FIG. 4, the beam tilting direction is describedas being one in number but there is available a technique forstructuring a three-dimensional image by irradiating an electron beam inat least two directions to form two images and superimposing theseimages. In this case, the steps S0004 to S0011 of FIG. 4 are executed inrespect of the respective tilt angles, coincident points betweenobtained two images are found and the two images are synthesized suchthat the coincident points overlap with each other. In structuring athree-dimensional image, techniques disclosed in embodiments 1 to 4 cancooperatively be used to structure a highly accurate three-dimensionalimage.

In case a plurality of sites at which three-dimensional images are to bestructured exist on the specimen, images may be acquired with a beamincident on the sites at one irradiation angle and thereafter images maybe acquired with a beam incident on the sites at a different irradiationangle. For example, images for templates of all of the plural sites arefirst acquired with a vertically incident beam and thereafter images ofall of the plural sites are acquired by giving the beam a tilt, thusobtaining images for beam condition adjustment and three-dimensionalimage structuring.

Through the process as above, time required for beam tilting can bereduced and when a plurality of measuring points exist, the overallmeasuring time can be shortened.

Embodiment 6

An example of preparing an expression of correcting the focus adjustmentamount and the FOV shift correction amount in relation to the tilt angleby measuring FOV shift amounts during electron beam tilting will bedescribed using a flowchart of FIG. 10. In step S0034, a top-down imagebefore beam tilting is first acquired. This top-down image is identicalto the image A explained in connection with embodiment 1. Next, focusevaluation is effected using the top-down image and focus adjustment forthe top-down image is carried out (S0035). Next, with the top-down imagekept in focus, the beam is tilted (S0036). With the beam tilted, thefocus is adjusted (S0037) and a FOV shift at that time is measured(S0038). This process is carried out for each beam tilt angle. FOV shiftamounts (inclusive of shift directions) and focus adjustment amounts arestored in respect of the individual tilt angles, thus ending the process(S0039). Subsequently, on the basis of differences of focus adjustmentamounts and differences of FOV shift amounts for the individual tiltangles, correction expressions are introduced, individually (S0040).

As described above, the expressions of correcting the focus adjustmentamounts and FOV shift amounts in relation to beam tilt angles areprepared, so that even when the beam is tilted at a desired angle(S0041), a focus adjustment amount and a FOV shift amount which conformto the angle can be calculated to effect the focus adjustment and FOVshift correction (S0042, S0043).

The description of the foregoing embodiments of this invention has beengiven by way of example of the scanning electron microscope but this isnot limitative and the present invention can also be applicable toanother type of charged particle beam apparatus such as an ion beamapparatus.

Aspects of the present invention can be set forth as below:

1. A charged particle beam apparatus having a charged particle beamsource, an objective lens for focusing a charged particle beam emittedfrom the charged particle beam source so as to irradiate it on aspecimen, a detector for detecting charged particles emitted from thespecimen, and a deflector for deflecting the charged particle beam to aregion which is out of axis of the objective lens and tilting thecharged particle beam in relation to an optical axis of the objectivelens,

the apparatus comprising a controller for preparing a template from animage formed under irradiation of the charged particle beam along theobjective lens optical axis, comparing the template with an imageacquired when the charged particle beam is tilted in relation to theobjective lens optical axis and detecting a shift between a field ofview before tilting of the charged particle beam and a field of viewafter tilting of the charged particle beam.

2. A charged particle beam adjusting method for correcting, when acharged particle beam is irradiated while being tilted in relation to anoptical axis of an objective lens, an astigmatic aberration of thecharged particle beam by using a stigmator capable of adjustingastigmatism correction intensities in a plurality of directions,comprising the steps of:

acquiring images in respect of individual combinations of adjustmentintensities in the plural directions;

determining evaluation values of the respective images; and

settling a combination of adjustment intensities of the stigmator on thebasis of a combination of adjustment intensities for which theevaluation value is high.

3. A charged particle beam adjusting method according to above 2,wherein the image acquisition is effected within a predetermined rangeof magnification.

4. A charged particle beam apparatus having a charged particle beamsource, an objective lens for focusing a charged particle beam emittedfrom the charged particle beam source so as to irradiate it on aspecimen, a detector for detecting charged particles emitted from thespecimen, a deflector for deflecting the charged particle beam to aregion which is out of axis of the objective lens and tilting thecharged particle beam in relation to an optical axis of the objectivelens, and a stigmator capable of adjusting astigmatism correctionintensities in a plurality of directions,

the apparatus comprising a controller for acquiring images in respect ofindividual combinations of astigmatism correction intensities in theplural directions, determining evaluation values of the respectiveimages and settling a combination of adjustment intensities of thestigmator on the basis of a combination of adjustment intensities forwhich the evaluation value is high.

5. A charged particle beam adjusting method for adjusting conditions fora charged particle beam when irradiating the charged particle beam whilegiving it a tilt in relation to an optical axis of an objective lens,

wherein when the charged particle beam is irradiated while being tiltedto the objective lens optical axis, focus adjustment is carried outafter the beam tilting and thereafter FOV shift correction is made.

6. A charged particle beam apparatus having a charged particle beamsource, an objective lens for focusing a charged-particle beam emittedfrom the charged particle beam source so as to irradiate it on aspecimen, a detector for detecting charged particles emitted from thespecimen, and a deflector for deflecting the charged particle beam to aregion which is out of axis of the objective lens and tilting thecharged particle beam in relation to the objective lens optical axis,

the apparatus comprising a controller for controlling, when the chargedparticle beam is irradiated while being tilted in relation to theobjective lens optical axis, the objective lens and the deflector suchthat focus adjustment by means of the objective lens is carried outafter the beam tilting and thereafter FOV shift correction is made.

7. A charged particle beam adjusting method for evaluating, from animage obtained when a charged particle beam is irradiated on a specimen,focus of the charged particle beam and/or FOV shift, comprising thesteps of:

measuring focus adjustment amounts and/or FOV shift amounts fromindividual plural images acquired when the charged particle beam isirradiated on the specimen in a plurality of tilting directions; and

preparing expressions of correcting the focus adjustment amounts and/orFOV shift amounts on the basis of the measured focus adjustment amountsand/or the measured FOV shift amounts.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1-9. (canceled)
 10. A charged particle beam adjusting method forperforming astigmatism correction of a charged particle beam using astigmator capable of adjusting intensities of astigmatism correction ina plurality of directions when the charged particle beam is irradiatedwhile being titled in relation to an optical axis of an objective lens,comprising the steps of: acquiring images corresponding to individualcombinations of the intensities of astigmatism correction in theplurality of directions; determining an evaluation value of each of theimages in respect of the individual combinations; and determiningcombination of intensities of astigmatism correction to be set by thestigmator on the basis of a combination of intensities of astigmatismcorrection for which the evaluation value is high.
 11. A chargedparticle beam adjusting method according to claim 10, wherein the imagesacquiring step is executed within a predetermined range ofmagnification.
 12. A charged particle beam apparatus comprising: acharged particle beam source; an objective lens for focusing a chargedparticle beam emitted from said charged particle beam source so as toirradiate it on a specimen; a detector for detecting charged particlesemitted from the specimen; a deflector for deflecting the chargedparticle beam to a region which is out of axis of said objective lensand tilting the charged particle beam in relation to an optical axis ofsaid objective lens; a stigmator capable of adjusting intensities ofastigmatism correction in a plurality of directions; and a controllerfor acquiring images corresponding to individual combinations ofintensities of astigmatism correction which are adjusted in theplurality of directions, determining evaluation values of the imagescorresponding to the individual combinations, and setting a combinationof intensities which are adjusted by the stigmator on the basis of acombination of intensities for which the evaluation value is high.
 13. Acharged particle beam apparatus according to claim 12, wherein saidcontroller acquires the images within a predetermined range ofmagnification.
 14. A charged particle beam apparatus according to claim12, wherein: said controller acquires the images corresponding toindividual first combinations of intensities of astigmatism correctionin one direction of X-axis and intensities of astigmatism correction inanother direction of Y-axis, on a two-dimensional region formed by theX-axis and the Y-axis, and when a combination of the first combinationsof intensities for which the evaluation value is high exists at an edgeof the first combinations, said controller sets a combination ofintensities on the basis of a plurality of combinations on thetwo-dimensional region in which a center combination of the plurality ofsecond combinations corresponds to the edge of the first combinations.15. A charged particle beam apparatus according to claim 14, whereinsaid controller executes additional acquisition of an image of acombination in the second combinations in which any images are notacquired.
 16. A charged particle beam apparatus according to claim 12,wherein said plurality of directions are two directions of a X-directionand a Y-direction.